The z-index property sets the stacking order for a group of elements whose x/y coordinates
overlap the same area.

Elements can have 3-dimensions.
The x- and y-components set the element position as viewed on the monitor.
The z-component determines which elements can appear to lie on top of other elements, or conversely,
which elements appear to lie under other elements.

The higher (or more positive) the number, the higher the element is in the stacking order.
Higher numbered elements overlay lower numbered elements (2 overrides 1).

auto

The auto value sets the stacking order number to the value of the parent element.

number

The number value can be zero, a positive integer, or a negative integer.
This sets the local stacking order.
An element with a higher number will be in front of all elements with lower numbers.

In this example, the blue colored word Dev has a higher z-index and is superimposed on top
of the red colored word Guru.